In ophthalmology, different methods have been developed for treating a visual deficiency in a patient by modifying the cornea of the patient's eye concerned. For instance, by means of laser radiation, the curvature of the cornea of a human eye can be selectively modified. Examples include known methods referred to by the acronyms LASIK, PRK and LASEK. In these methods, a treatment laser beam is passed, in a scanning mode, over the pupil area to be corrected, said laser beam thus causing a modification of the cornea. This modification of corneal geometry has to be effected at determined positions relative to the visual axis of the eye so as to allow said visual deficiency to be improved or removed.
However, during the treatment period, the eye executes a multiplicity of voluntary and involuntary movements, e.g. saccades, microsaccades, torsional movements, etc., which are accompanied by a corresponding movement of the visual axis. However, during treatment, such movements prevent precise alignment of the treatment laser beam relative to the visual axis.
Various methods are used to reduce or to prevent such deviations.
A first class of methods attempts to completely suppress any movements of the eye. For this purpose, for example, so-called applanation objects, e.g. plates or curved contact glasses, which are connected to the treatment device, can be employed, which are retained on the front section of the eye by mechanical pressure and/or vacuum. Due to the mechanical coupling of the eye with the treatment device, the eye movement relative to the treatment device is suppressed. Therefore, the treatment laser beam can be precisely aligned relative to the visual axis of the eye. However, the use of such applanation objects is, in many cases, undesired.
In a second class of methods, the effects of the eye movement on the alignment of the treatment laser beam relative to the visual axis are compensated for by selective and near-simultaneous re-direction of the treatment laser beam according to the eye movement. This requires detection of the movement of the eye.
In most cases, the detection of movement is based on video detection of the front eye section and subsequent digital image processing and evaluation. This allows detection of typical characteristics of the eye, e.g. the pupil rim or the transition between the iris and the sclera, and determination of their movement and position.
In further methods for the detection of movement, the pupil rim, the sclera boundary or artificially applied marks, as in EP 125 28 72, are scanned.
From the determined position and movement data, a compensation signal is then generated, which is used for positioning the treatment laser beam.
However, the video-based methods have the disadvantage that the movement and position signals are generated at insufficient speed or frequency, respectively. Said methods do not allow tracking of rapid eye movements, so that, where high precision is required for aligning the treatment laser beam relative to the visual axis of the eye, considerable deviations may occur between the desired position of the treatment laser beam and the actual position of the treatment laser beam relative to the visual axis of the eye.
Moreover, movement of the eye is only detected in two spatial dimensions, which are essentially perpendicular to the visual axis of the eye.
Therefore, it is an object of the present invention to provide a method and a device for determining eye movement, said method or device allowing quick, high-precision determination of the eye movement.